Particulate materials such as finely ground, or comminuted, coal or shale, or catalyst particles are frequently fluidized for flow through conduits by introducing aerating gas, such as steam, air or the like, at locations spaced apart along the conduit. Generally, the flow of gas is at a pressure and in sufficient volume to achieve an average density of the mixture of particles and gas so that their flow is fluid-like. That is, the mixture is sufficiently homogeneous so that it acts as a fluid as it passes through the conduit.
In fluid catalytic cracking processes, finely divided catalyst particles are generally transported by a combination of gravity and steam flow. Steam flow both levitates and controls the direction of flow through a reactor riser wherein a hydrocarbon feed stream is added to the so fluidized catalyst particles after they have been heated. In a cyclic system, heat is usually added to the catalyst by burning "coke" from "spent" (already reacted) catalyst in a regenerator vessel. After reaction of the hydrocarbonaceous material with the hot fluidized catalyst in a reactor pipe, the mixture is separated into its gaseous and solid phases in a separator vessel. Hydrocarbon vapor is recovered overhead in the separator vessel and then distilled. Spent catalyst is returned from the separator vessel to the regenerator vessel through a stripping column in which an inert gas (steam usually) is flowing. Stripped catalyst flows through another conduit to the regenerator as a fluidized mixture produced by levitating steam. The pressure and volume of the aerating gas, control both the composition of the fluidized mixture and the circulation rate of catalyst particles returning to the regenerator for the next cycle.
Where a regenerator vessel and a separator vessel are arranged side-by-side, the two conduits are usually in the form of large U-tubes which have central sections that are substantially horizontal. The direction of flow changes in this central section from down to up in going from the regenerator into the riser-reactor. A similar change in direction is required in the other U-tube for spent catalyst to return for regeneration from the separator-stripper.
Generally aerating steam is admitted to the conduits, (formed by either the riser pipe or the spent catalyst return pipe), at a plurality of axially spaced apart locations. A plurality of nozzles enter the conduit radially to form an internal gas ring around the inner pipe wall. The nozzles are generally fed by a steam manifold encircling the pipe at each location. Sometimes the radial nozzles at more than one location are connected to a single common manifold.
In general, it is expected that gas from the nozzles at each axial position will uniformly mix with the particles, which typically have a diameter of 20 to 100 or more micrometers (microns) to form a homogeneous mixture of particles in a uniform dispersion of small gas bubbles. In practice it is customary to supply more steam than is required to achieve such homogeneity, because too little gas flow may result in no net movement of particles along the conduit. In the extreme, catalyst may "slump" to the low side of the conduit and result in two separate phases, solid and gas, with only the surface of slumped particles moving with the gas, a "sand dune" effect.
On the other hand, too much gas flow may produce unstable or inadequate catalyst flow with the gas with or without creation of bubbles which grow and collapse. This results in both reduced product yields from the system and higher operating costs because of uneconomical use of compressors or steam generators to provide aeration gas.
In severe cases, bubble growth and collapse may also be rapid and result in extreme vibration of the U-tube conduits and the interconnected regenerator, reactor and stripper vessels. Since in commercial refineries these vessels are generally supported at levels of 60 to 100 feet above ground, undue mechanical vibration may pose a serious hazard to the entire system.
Accordingly, in all but the uniformly dispersed, small bubble case, inadequate and uneconomic catalyst flow results and serious mechanical problems can result. For these reasons, there has long been a need for accurate control of the aeration gas for more efficient use of the gas and to optimize both the composition of the fluidized mixture and the rate of flow of a uniform dispersion of catalyst particles in the fluid stream.
In general, fluid catalytic cracking units operate at temperatures of about 950.degree. F. to 1000.degree. F. for the reactor riser and up to about 1250.degree. F. in the regenerator. Accordingly, the catalyst temperatures in both of the U-tubes supplying fresh catalyst and returning spent catalyst are usefully kept well above about 900.degree. F. during continuous recirculation. For this service, the steel conduits forming the U-tubes have walls that are on the order of an inch thick. Typically, they are several feet in diameter. The interior surface of the conduits is coated with a ceramic material both to insulate the steel and to prevent abrasion by the fine catalyst. The normal rate of flow of such catalyst is in the range of 30 to 60 tons per minute and the resulting average velocity of the catalyst particles is on the order of 10 to 30 feet per second and gas velocity is on the order of 30 to 60 feet per second through the conduits.
While it has been known heretofore to measure bulk density of materials with electromagnetic radiation, for example X-rays, gamma-rays and neutrons, measurement of, and use of such measurement to control both composition and rate of flow of comminuted particles in an aerated stream has not been known or used. Specifically, it has not been known or suggested heretofore to control a fluid catalytic cracking system, by irradiating a flowing stream of a mixture of particles, such as catalyst, and aerating steam with penetrative radiation, such as gamma rays, detection of the absorption of such gamma rays by the flowing stream of a mixture of such catalyst particles and steam, and then regulating steam supplied to the conduit in an amount and to an extent required to control fluctuations in the composition and rate of flow of the mixture in response to said gamma ray absorption.